1. Field of the Invention
The invention is a method of stabilizing wastewater treatment sludge (WWTS) by treating it with quick lime and fly ash to produce a stabilized composition that meets the U.S. Environmental Protection Agency (EPA) process for further reduction of pathogens (PFRP) standard. This stabilized material can be utilized as sanitary landfill cover material. The invention provides an environmentally sound method of reclaiming fly ash and sludge and converting these waste products into a useful end product.
2. Description of the Related Art
An urban, industrialized society produces large quantities of waste. Some of this waste is in the form of waters containing waste such as organic-laden sewage waters treated at municipal, commercial and industrial water treatment works, these wastewaters contain a variety of organic and inorganic compositions that are often treated onsite. Depending upon the treatment process, slurries high in suspended solids may be formed. These slurries, commonly referred to as sludges, are produced either by the concentration of solids originally in the waste water (e.g. primary sludge) or the formation of new suspended solids which result from the removal of dissolved solids from the waste water (primary and tertiary sludge).
The term "wastewater treatment sludge (WWTS)" as used in the specification and claims includes sludges produced from the treatment of organic-laden sewage waters such as the sludges produced by municipal, industrial and commercial wastewater treatment facilities, and the like. Thus, WWTS includes sludges produced from sewage subjected to primary treatment, sludges from sewage subjected to secondary treatment, and sludges subjected to tertiary treatment.
In some cases these sludges are digested or otherwise stabilized at the wastewater treatment facilities. In other cases the sludges may not be stabilized prior to disposal. In either case, these sludges may be or may become odoriferous and may contain a broad variety of bacteria, parasites, viruses and other pathogenic organisms.
Sludges that contain elevated levels of heavy metals are precluded or seriously inhibited from land application. Since large volumes of non-hazardous sewage sludge are produced and beneficial reuse of a large proportion of this sludge is inhibited due to metals and pathogen content, odor problems and seasonal restrictions on land application or land availability, it would be desirable to develop methods for converting these sludges into an odorless, stabilized end-product that is needed year round.
Coincidentally, large quantities of fly ash from the combustion of coal, lignite or wood are generated in this country. Fly ash is utilized as an additive in concrete, as a soil stabilization material or as a soil additive; however, large volumes are disposed of in sanitary landfills or abandoned mining excavations. These disposal methods may lead to ground water contamination, or consume valuable disposal capacity within sanitary landfills. Therefore, it would be desirable to develop additional methods that provide long term environmentally sound methods of utilizing fly ash in a beneficial manner that minimizes the consumption of landfill space.
The U.S. Environmental Protection Agency (EPA) has recognized the problems which disposal of pathogen infested waste can present. In response to these problems, the EPA has issued guidelines at 40 C.F.R. .sctn.257 concerning land disposal of sewage sludge which contains pathogens. The EPA regulations recognize three separate categories of sludge: unstabilized sludge; sludge exposed to a process to significantly reduce pathogens (PSRP); and sludge exposed to a process to further reduce pathogens (PFRP).
"Unstabilized sludge" has not been exposed to any pathogen reducing process, is not suitable for general land application, and can only be incinerated or landfilled. Sludge which undergoes a PSRP, such as an anaerobic digestion, heat treatment, lime stabilization, or air drying, can be disposed of on land only if public access to the land is controlled for a period of from 12 to 18 months. Finally, sewage which has undergone a PFRP has no disease-related restrictions on reuse. Unfortunately, known PFRP methods are expensive, highly energy intensive processes, such as irradiation or thermal processing, which are unsuitable for transforming sewage sludge into a readily usable end product.
If a treated sludge is to be readily usable, it is desirable that it meet the EPA's PFRP standard so that it poses a minimal risk of transferring diseases to humans. Further, it must not attract vectors and it must not be flammable. Also, if it is to be disposed of as landfill cover material, it must be readily spreadable with conventional equipment. In addition, it must also have a load bearing capability of at least one ton per square foot and have a permeability in the range 10.sup.-5 cm/sec to 10.sup.-6 cm/sec.
The early prior art recognizes the need to concentrate the waste contained in wastewaters for ease of treatment and disposal. For instance, U.S. Pat. No. 1,672,584 relates to the treatment of sewage with an alkaline composition, such as hydrated lime, to change the positively charged colloids, that make up the organic waste, into negatively charged colloids. These negatively charged colloids are then coagulated with an electrolyte, such as iron or aluminum sulfate. As an alternative, cement kiln dust could be used as the electrolytic coagulant. The high density of cement kiln dust is said to materially increase the settling rate of the precipitate formed.
U.S. Pat. No. 3,476,683 describes mixing sludge with lime. Heat is generated during this mixing due to the reaction of lime and water and the temperature increases to "more than 100.degree. C." The sludge powder obtained is said to be "sterilized" by the heat generated.
Both of the above patents recognize the need to use inexpensive reagents, both use lime. Since large volumes of sludge must typically be treated, it is economically desirable to use inexpensive, readily available reagents to treat or stabilize the sludge. It is even more desirable to combine other waste products with the sludge if one could thereby both stabilize the sludge for disposal and dispose of the other waste products simultaneously.
U.S. Pat. No. 4,306,978 relates to the use of lime for the treatment of sludge to produce stabilized sludge pellets. A dewatered cake containing 10-60 wt. % of dry solids is mixed with 2-40 wt. % of lime, based on the dry solids weight. During this process, the temperature increases to the range 95.degree.-100.degree. C. due to the chemical reaction between lime and water. Further, this high temperature coupled with the high pH caused by the lime, is said to destroy bacteria and pathogenic organisms in the sludge. This treated mixture is then mechanically pelletized.
U.S. Pat. No. 4,226,712 is also directed to the treatment of sludge with lime to obtain a partially dried mixture. This mixture is then subjected to a second step of drying and granulation to produce a product suitable for use as fertilizer.
Similarly, U.S. Pat. No. 4,781,842 relates to a two-step process for treating sludge with lime, cement kiln dust, or lime kiln dust to raise the pH above 12 for at least about 2 hours and then drying the resulting mixture by an aeration process to produce a granular product. This process is said to meet USEPA PFRP standards and its product may be used as a fertilizer. U.S. Pat. No. 4,554,002 uses a similar process and suggests that fly ash may be substituted for cement kiln dust.
U.S. Pat. No. 4,793,927 is directed to a method of encapsulating sewage in an "impervious mass" with an "ammonia source" so that the ammonia released within the mass will destroy helminths, an organism that is difficult to destroy by other methods. The end product is a friable, clay-like material. The process meets EPA PFRP standards and the product is suitable for use as landfill cover, land reclamation material, fertilizer, etc. The method includes mixing the sewage with a setting agent and silicate in the volume ratio about 10:1:0.5. Suitable setting agents include Portland cement, lime and kiln dust. The silicate used is sodium silicate.
Fly ash is solid waste material that is readily available and that would be attractive to use as a reagent to treat and stabilize WWTS. Fly ash is produced as a residue of the combustion of coal, lignite, or wood and is an artificial pozzolan.
Early efforts to dispose of fly ash included combining dry fly ash and hydrated lime to produce a hydrated lime cement. U.S. Pat. No. 2,564,690 describes how to make such a cement and mix it with an aggregate and water to produce a composition capable of setting up to a high strength. U.S. Pat. No. 3,852,084 addresses the slow hardening of lime-fly ash cements and teaches milling of the lime and fly ash to increase their reactivity and increase the rate of hardening.
U.S. Pat. No. Re. 30,943 and U.S. Pat. No. 4,101,332 describe the use of a mixture of fly ash and cement kiln dust together with an aggregate, such as limestone, sand, blast furnace slag, gravel and the like, as a road stabilizer base. Thus, the mixture sets up to a hardened, load-bearing mass. Similarly, U.S. Pat. No. 4,038,095 is directed to a load bearing composition that includes fly ash, lime stack dust and an aggregate.
U.S. Pat. No. 4,432,800 is directed to a method of producing a hardened durable mass using beneficiated kiln dust which typically has low reactivity. The reactivity of the kiln dust is increased by the addition of lime, calcium hydroxide, or sodium hydroxide. The beneficiated dust can be used in conjunction with fly ash and an aggregate to produce a hardened durable mass.
The related art shows methods for concentrating sludges and methods for stabilizing sludges. Some of these methods do not meet the PFRP standard, indeed, only the methods of U.S. Pat. Nos. 4,793,927; 4,781,842 and 4,554,002 are said to meet this standard. Methods of these patents produce a friable, clay-like product.
U.S. Pat. No. 4,028,130 discloses a method of using digested sewage sludge producing a "hardenable composition" suitable for disposal in landfills. This method entails mixing the sewage sludge with hydrated lime and fly ash in the proportions (based on wt. % solids) 5-50% water, 1-15% hydrated lime, 1-50% sludge solids, 20-90% fly ash, 0-60% soil and 0-10% calcium sulfate. The patent does not mention the EPA's PFRP standard or that its hardenable composition meets this standard.
What is yet needed is a method of stabilizing WWTS to meet PFRP standards that utilizes readily available and inexpensive reagents to produce a solid material, either friable and clay-like, or a solid mass capable of supporting at least a low load, depending upon the quantity of reagents used, and that can be utilized safely and effectively as landfill cover.